The present invention relates to a method and apparatus for controlling a single crystal grown by the Czochralski method and forming said crystal into a predetermined shape.
In growing single crystals of good quality by the Czochralski method, the diameter of the crystal's body must remain constant. In addition, the shoulder section of the crystal must have a desired shape. This is particularly important in preventing strain and dislocation, as well as the formation and propagation of twins. In particular, with a III-V compound semiconductor single crystal having a zinc blend structure, many reports have been made on the shape of the crystal shoulder section and, in particular, on the relationship between the angle of the crystal shoulder section and the formation of twins. Such a report is made in, for example, W. A. Bonner: Mat. Res. Bull. 15 (1980), 63.
The crystal differential weight at the shoulder section is not constant and changes over time. Accordingly, shape control of single crystals has been conventionally effected as follows. When a single crystal having the shape shown in FIG. 1 is to be grown, its weight changes both linearly and nonlinearly, as shown in the graph of FIG. 1. The crystal weight is a cubic function of time in the nonlinear region and is a linear function of time in the linear region. To obtain the reference weight signal given in the nonlinear and linear regions, Japanese Patent Publication No. 54-4345 discloses a method for directly moving a potentiometer along a plate having a shape corresponding to the reference weight curve. Japanese Patent Publication No. 54-4345 also discloses a method of obtaining a reference weight signal by using a programmer for moving a potentiometer along a plate having a shape corresponding to a time differentiated curve of the reference weight, and by rotating a motor in such a way as to move a heliohm. Another method is disclosed by A. E. Zinnes and B. E. Nevis in "Automatic Diameter Control of Czochralski Grown Crystals" (J. Crystal Growth 19 (1973), pp. 187 to 192). According to the method of this literature, a time differentiated function of the reference weight of the crystal shoulder section is computed by a computer, as a linear function of time, and the obtained function is processed by software. By either of these methods, the reference signal is obtained as a function of time. Furthermore, these methods are both applied to growing oxide single crystals.
In the actual manufacture of single crystals, the shoulder section does not always grow uniformly after a seed crystal is dipped into a melt. For example, if the temperature at which the seed crystal is dipped into the melt is too high or too low, the pulling of the seed crystal may not easily result in the formation of crystals, or may result in the rapid growth of crystals. Accordingly, when the shape of a single crystal is controlled by the conventional method, a great offset tends to be generated. When intense control is performed to prevent such an offset, the crystal has an irregular shape at the shoulder section, resulting in an increased strain or the formation of twins. Conversely, when such an offset is allowed to remain after the control operation, deviation of the crystal shape from the desired shape increases and manufacturing precision is lowered. This is particularly so in the manufacture of a III-V compound single crystal. Thus, conventional methods do not allow for the stable formation of the shoulder section of a III-V compound single crystal of high quality. Therefore, conventional methods cannot be applied to the manufacture of III-V compound single crystals having a large diameter and a large volume, such as InSb, GaAs and InP, which are receiving attention as single crystals suitable for use in infrared ray CCDs, ultra high-speed ICs, and FETs.